The hip joint has been prosthesized for more than forty years by prosthetic bodies apt to reproduce the coupling between the femoral head and the acetabular cup. These bodies work in an environment encapsulated by the so-called sinovial membrane, which contains a viscous liquid lubricant consisting of plasma, water, salts and hyaluronic acid, called sinovial liquid, apt to ease the sliding of the contacting articular surfaces. Therefore, the relative motion of the latter takes place under aqueous lubrication conditions.
In the normal natural biological system, the friction coefficient between the articular surfaces is usually comprised in a range of values of about 0.005 to 0.025. However, notwithstanding the intensive research in the field, the current prosthetic systems fail to reproduce the performances of the natural system, due to the difficulties of ensuring the setting up of a supporting and stable meatus of sinovial liquid between the prosthetic bodies.
The main problem is that of the wear detritus, often referred to as ‘debris’, which are one of the main causes for long-term failure of the orthopedic prostheses in general and of the hip prostheses in particular. In fact, debris can get stuck between the two prosthetic bodies, favoring the three-body wear mechanism, or deposit onto the femoral surface near to the prosthetic stem.
This problem is particularly felt just in the case of the prosthetic systems to date most widely adopted, which employ high-molecular weight polyethylene, often indicated with the acronym UHMWPE, for the manufacture of the acetabular cup and of a Chromium-Cobalt (CoCr) alloy, of AISI 316 steel or of a ceramics material for the manufacture of the femoral head. In fact, polyethylene, being the softer material of the coupling, tends to form debris which accumulate in correspondence of the prosthetic stem, so inducing macrophages to migrate thereat, i.e. causing a reaction by the organism which tends to eliminate the debris as extraneous particles. Where the polyethylene debris have relevant dimensions, the macrophages aggregate, forming polynucleated giant cells. If, as it often happens, the macrophages fail to eliminate the debris as the latter are too much and/or too large, an inflammatory process sets up. Moreover, the aggregated macrophages induce the production of substances activating the osteoclastic reaction and therefore the degradation of the bone tissue at the proximal region of the stem. Apparently, these circumstances concur to modify the distribution of the mechanical load onto the prosthesis, unbalancing it, with a consequent danger of mobilizing the latter.
Examination of explanted hip prostheses suggested that the debris problem could be almost completely solved by the use of a coupling between prosthetic bodies both made of ceramics. However, the latter material, although not producing debris, causes an excessive rigidity of the prosthetic system, the bodies of which will scarcely be able to adjust their mutual location after the implant.
The technical problem underlying the present invention is to provide a hip prosthesis and designing method thereof which allow to overcome the drawbacks hereto mentioned with reference to the known art.
This problem is solved by a hip prosthesis, comprising a first prosthetic body reproducing an acetabular cup and a second prosthetic body reproducing a femoral head and apt to be coupled to said first prosthetic body, characterized in that said prosthetic bodies have low debris production and in that said first prosthetic body has a wettability substantially different from that of said second prosthetic body.
According to the same inventive concept, the present invention further relates to a designing method of a hip prosthesis comprising two prosthetic bodies reproducing an acetabular cup and a femoral head, respectively, characterized in that it provides that said prosthetic bodies have a low debris production and a substantially different wettability.
As it will be made apparent from the description which follows, by “substantially different wettability” it is meant that the two prosthetic bodies must have appreciably different wettability properties, e.g. having one an essentially hydrophilic and the other one an essentially hydrophobic behavior, or the one a slightly hydrophilic and the other one a markedly hydrophobic behavior, or vice versa, and so on. In this condition, a lubricated system ensues which, as such, does not produce debris.
The present invention provides several relevant advantages. The main advantage lies in that said opposite wettability of the prosthetic bodies enables the setting up of a markedly stable supporting meatus therebetween.